Living bone tissue exhibits a dynamic equilibrium between formation of bone, known as deposition, and breakdown of bone, known as resorption. These processes can be mediated by at least two cell types: osteoblasts, which secrete molecules that comprise the organic matrix of bone (deposition); and osteoclasts, which promote dissolution of the bone matrix and solubilization of bone salts (resorption). In certain individuals, such as post-menopausal women, the rate of resorption can exceed the rate of deposition, which may result in reduced bone mass and strength, increased risk of fractures, and slow or incomplete repair of broken bones.
Osteoprotegerin ligand (OPGL) is a member of the TNF family of cytokines and promotes formation of osteoclasts through binding to the receptor activator of NF-κB (RANK, also called osteoclast differentiation and activation receptor, or ODAR). Osteoprotegerin (OPG), on the other hand, inhibits the formation of osteoclasts by sequestering OPGL and preventing OPGL association with ODAR. Thus, the amount of OPGL associated with ODAR correlates with the equilibrium between bone deposition and resorption. Individuals who suffer from osteopenic diseases, such as osteoporosis, show a greater rate of bone resorption than deposition, which may result from increased levels or activity of OPGL. Thus, it would be useful to have molecules that can regulate the activity of OPGL in osteoclastogenesis. It would also be useful to be able to detect the amount of OPGL in a biological sample, such as a blood sample, to diagnose an osteopenic disorder relating to increased levels of OPGL.